1. Field of the Invention
The invention relates to a circuit arrangement for creating microwave oscillations having an electronic oscillator with two transistors and a resonator, preferably with a base impedance network and with two emitter impedance networks, wherein the base impedance network and/or an emitter impedance network or both emitter impedance networks can be put out of tune, wherein the transistors each have a parasitic base collector capacitance and wherein a compensation capacitance is switched between each the collector of the first transistor and the base of the second transistor as well as the collector of the second transistor and the base of the first transistor.
2. Description of Related Art
A circuit arrangement of the type to which this invention is directed is known from the German Patent Application 10 2008 061 254. The creation of microwave oscillations, i.e., of high frequency to highest frequency electric oscillations, for example, up to about 80 GHz or even up to 100 GHz, is central to many different applications. In this manner, extremely stable microwave oscillations are required by, e.g., radar systems for measuring distances and for measuring speeds. Such microwave oscillations are also needed for other measuring devices and for communication applications.
An electronic oscillator is normally used for creating microwave oscillations, which can be implemented either as a monolithic integrated circuit on a chip or as a hybrid circuit on a printed circuit board. According to Barkhausen's formula, these electronic oscillators—as all electronic oscillators—have at least one amplifier element, in particular—for decades—a semiconductor device as amplifier component. In particular, the amplifier element can be a transistor—of any type. A resonator further belongs to the electronic oscillator—again as all electronic oscillators—that determines the frequency of the electronic oscillator. At least one tunable impedance is necessary for electronic oscillators, in which the frequency of the microwave oscillations can be set. A tunable capacitance, a varicap, is often used as tunable impedance. By changing the control voltage on the varicap, the relevant capacitance changes and, thus, also the frequency of the electronic oscillator. In the ideal case, the frequency of the electronic oscillator is only changed via the control voltage on the varicap and the frequency of the electronic oscillator does not depend on further factors. In reality, however, further factors influence the behavior of the electronic oscillator, in particular the frequency of the electronic oscillator, such as the supply voltage of the electronic oscillator, the current temperature or the outer load on the electronic oscillator. This influence can be partially compensated with phase control. However, the influence of the outer load on the electronic oscillator, the load repercussion, cannot be compensated with phase control.
An insufficient decoupling of the electronic oscillator from the outer load results in that the outer load, which, for example, can be a longer connecting line, additionally acts as a resonator for the electronic oscillator. This parasitic resonator can, in many cases, have a substantially better quality factor than the actual resonator of the electronic oscillator. However, this improvement of the quality factor is only effective at certain frequencies, at other frequencies the overall quality factor of the electronic oscillator is considerably downgraded by the external resonator. Consequently, a modulation of the phase noise results via the frequency of the electronic oscillator. In extreme cases, the tuning characteristic of the frequency of the electronic oscillator can exhibit skips via the control voltage. A small change in the control voltage can lead to a sudden skip of the frequency of the electronic oscillator. This can go so far that the tuning characteristic is not unambiguous, so that tuning in different direction leads to a hysteresis in the tuning characteristic. As a result, the electronic oscillator can no longer be tuned smoothly. In addition, a sudden switch of the load, for example, by switching between different antennae, can put the electronic oscillator out of tune. However, even in electronic oscillators having phase control these skips as well as too strong phase noise can lead to the phase control no longer being capable of stabilizing the electronic oscillator, so that the entire circuit arrangement is no longer functional. In order to prevent this effect, which is called “load-pulling” in technical literature, a stronger decoupling of the electronic oscillator from its outer load is necessary. Insofar, there are three types of solutions that are often combined for use:
The first solution is a passive, reciprocal decoupling. Here, a passive, reciprocal decoupling network is switched between the load and the electronic oscillator. This can, for example, be implemented with a damping member, with a voltage divider or with a simple resistor switched in series or parallel to the load. A passive decoupling always results in a reduction of the available output lines of the electronic oscillator, since the reverse isolation of the passive, reciprocal decoupling network is always identical to the damping of the output signal.
A second solution is a passive, non-reciprocal decoupling. Here, in turn, passive decoupling networks are used, however with non-reciprocal components such as isolators, circulators or ferrites. With these components, it is possible to achieve a reverse isolation, which can be considerably greater than the damping of the output signal of the electronic oscillator. The output line of the electronic oscillator remains essentially intact. However, non-reciprocal components can only be implemented with considerable effort. Integration on one chip is not possible with reasonable effort, integration on a printed circuit board is costly.
The third solution is active decoupling. Here, an active circuit of semiconductor devices, e.g., transistors, is switched between the load and the electronic oscillator. This circuit often even acts as an amplifier; for this reason, the term separation amplifier is often used. These circuits can achieve a large reverse isolation without damping the output signal of the electronic oscillator. However, additional power is principally needed for operating an active decoupling, which then increases the overall power loss. In many applications, the power consumption of the separation amplifier exceeds that of the actual electronic oscillator, since the separation amplifier is run in multiple steps for good decoupling.
In order to implement electronic oscillators in the millimeter wave range, monolithic integrated circuits are often used. In these circuits, a complete symmetrical structure is usually chosen. This is referred to as a differential circuit design. This prior art will be described in the following in conjunction with FIG. 1; here, FIG. 1a) shows the principal and FIG. 1b) a typical implementation.
FIG. 1 shows an electronic oscillator 1 of a circuit arrangement for creating microwave oscillations having two amplifier elements, namely with two transistors 2 and 3 and with a resonator 4. In the electronic oscillator 1 shown in FIG. 1, the resonator 4 has a base impedance network 5 and two emitter impedance networks 6 and 7. It is not shown that the base impedance network 5, one emitter impedance network 6 or 7, both emitter impedance networks 6, 7 or the base impedance network 5 and the emitter impedance networks 6 and 7 can be tuned. The base impedance network 5 and the emitter impedance networks 6, 7 do not determine only the frequency of the electronic oscillator 1, they also determine the biasing of the transistors 2, 3. The base impedance network 5 and the emitter impedance networks 6 and 7 do not have to be implemented separately, a coupling is also possible. In order to fulfill the oscillation requirements, the base impedance network 5 should act primarily inductively and the emitter impedance networks 6, 7 should act primarily capacitively. A current has to be fed into the transistors 2 and 3 via the emitter impedance networks 6 and 7. When the oscillation requirements are fulfilled, this provides for a push-pull oscillation, i.e., the transistors 2 and 3 work shifted exactly half of a period.
In the implementation shown in FIG. 1b) of the electronic oscillator 1 shown as a principal in FIG. 1a), the base impedance network 5 consists of two inductances 8, 9, while both emitter impedance networks 6 and 7 consist of two capacitances 10, 11 and of two inductances 12, 13. The inductances 12, 13 are very large and act primarily to feed current into the transistors 2, 3.
In the embodiment shown in FIG. 1 of an electronic oscillator 1 belonging to a circuit arrangement for creating microwave oscillations, FIG. 1a) principal representation, FIG. 1b) typical representation, the output signal of the electronic oscillator 1 is decoupled via the collectors 14, 15 of the transistors 2, 3 and is typically led via a matching network (not shown) and an output buffer (not shown) to the load (also not shown). This decoupling via the collectors 14, 15 of the transistors 2, 3 causes a very good decoupling in the ideal case. However, the transistors 2, 3 each have a parasitic base collector capacitance (not shown). The parasitic base collector capacitances cause a strong repercussion of the load on the electronic oscillator 1, especially at high frequencies.
Based on the prior art described above in detail, there has been an effort to reduce, or as far as possible, to eliminate the described disadvantageous repercussion of the load on the electronic oscillator, and thus, on the circuit arrangement for creating microwave oscillations to which the electronic oscillator belongs. This has been attempted in that a compensation capacitance is switched between the collector of the first transistor and the base of the second transistor as well as between the collector of the second transistor and the base of the first transistor and the compensation capacitances are about as large as the parasitic base collector capacitances (compare the German Patent Application 10 2008 061 254). This has the effect that a signal, which is coupled by the load of the electronic oscillator, is eliminated to the greatest possible extent. The signal, which is coupled by the load of the electronic oscillator, is coupled at the same phase on both sides. Since the electronic oscillator oscillates push-pull, the signal coupled to the load in the electronic oscillator does not emerge in the differential signal, i.e., in the output signal.
The basic possibility for compensating the influence of the base collector capacitance has also already been described in literature (“Design and Scaling of W-Band SiGe BiCMOS VCOs” in the “IEEE Journal of Solid-State Circuits”, vol. 42, No. 9, September 2007). This compensation is also used in the prior art in order to compensate for the Miller effect, and thus, to increase the frequency of an electronic oscillator (compare here pages 316 and 317 in the standard volume “Halbleiter-Schaltungstechnik” by Dr.-Ing. Ulrich Tietze and Dr. Christoph Schenk, 12th Edition, Springer-Verlag and “Dependence of the input impedance of a three-electrode vacuum tube upon the load in a plate circuit” in “Scientific Papers of the Bureau of Standards” 15(351): 367-385, 1920). However, a Miller effect only occurs when a voltage amplifier occurs between the base nodes and the collector nodes of a transistor, which is not the case in many dimensions.
Due to the decreased repercussion of the load on the electronic oscillator, called load repercussion in the following, it is possible, compared to other circuit designs (compare “Fully Integrated SiGe VCOs With Powerful Output Buffer for 77-GHz Automotive Radar Systems and Applications Around 100 GHz” in “IEEE Journal of Solid-State Circuits”, vol. 39, No. 10, October 2004), to substantially simplify the output buffer and to reduce the required power dissipation, through which, inter alia, the effort for heat dissipation is considerably reduced and the reliability of the electronic oscillator—and thus the circuit arrangement according to the invention for creating microwave oscillations—can be increased. Especially in battery-operated circuit arrangements, the power provided is limited so that, in particular, the decrease in the required power dissipation has a positive impact.
The described reduction of the load repercussion reduces the load repercussion in a typical implementation of the reverse isolation, as described in the introduction with the term “load-pulling”, at a factor of 5 to 10.